Wafer-level chip-scale (WL-CS) packaging of cameras manufactured with CMOS technologies has contributed to the incorporation of cameras in high-volume consumer products such as mobile devices and motor vehicles. Such a camera includes a CMOS image sensor having a pixel array, wherein each pixel includes a microlens that focuses light on it. Contaminates incident on a microlens can adhere to it and render the associated pixel inoperable. Thus, one function of a WL-CS package is to isolate each CMOS image sensor on a wafer from said contaminates. Wafer-level packaging cannot contact the microlenses, for risk of damaging them, and the packaging must be optically transparent so as not to filter the light incident on the CMOS image sensor.
FIG. 1 depicts a carrier wafer 124 above a device wafer 122 that includes an array of dies 101 that each include a semiconductor device thereon. In the embodiment of FIG. 1, the semiconductor device is a CMOS image sensor that includes a microlens array 100. Device wafer 122 may be formed of silicon, silicon-germanium, silicon carbide, or similar materials used in the art. Carrier wafer 124 is part of a WL-CS package and may be formed of glass, plastic, or any material that does not impede the operation of the semiconductor devices on dies 101. In the embodiment of FIG. 1, carrier wafer 124 is optically transparent so as not to significantly filter light reaching microlens array 100.
Dicing an assembly that includes device wafer 122 encapsulated by carrier wafer 124 results in WL-CS packaged image sensors, such as a prior-art WL-CS packaged image sensor 290 of FIG. 2. FIG. 2 is a cross-sectional view of WL-CS packaged image sensor 290 disclosed in U.S. Pat. No. 6,777,767 to Badehi. A microlens array 207 is mounted on a substrate 202, which is connected to conductive pads 212. Conductive pads 212 are electrically connected to electrical contacts 208, which are electrically connected to a conducting bump 210. An epoxy 204 bonds substrate 202 to a bottom packaging layer 206. Spacer elements 216 separate a glass packaging layer 224 from microlens array 207, creating a microlens-array cavity 220. An epoxy sealant 218 seals cavity 220, which has a cavity height 221. In the direction of cavity height 221, epoxy sealant 218 bridges cavity 220 between packaging layer 224 and electrical contacts 208, and between packaging layer 224 and substrate 202.
A limitation with WL-CS packaged image sensor 290 is that the coefficient of thermal expansion (CTE) of conductive pads 212, spacer elements 216, and epoxy sealant 218 are sufficiently disparate that delamination can occur should temperature change during fabrication, for example, during the surface mounting process. U.S. Pat. No. 7,528,420 to Weng et al. addresses this limitation by applying recesses into a carrier wafer 324, illustrated in FIG. 3. Carrier wafer 324 is bonded to a device wafer 322. Carrier wafer 324 includes recesses 316 that result in microlens cavities 320 that each enclose a respective microlens array 307. Recesses 316 are photolithographically etched into carrier wafer 324. Each microlens cavity 320 has a cavity height 321.